The present invention is related to electronic switching circuits for controlling devices or systems connected to the circuit output, and more particularly, to electronic switching circuits receiving dual input signals, and to direct, in turn, a device or a system which often will be the primary device controlling power transfers in a power circuit containing a substantial power consuming load means.
In many circuit uses for electronic switching control circuits of the kind involved in controlling power transfers between a power source and a power consuming load, the control circuit will operate from a relatively low voltage, constant polarity power supply or supplies. On the other hand, the power circuit over which control is exercised will operate from a relatively high value voltage supply, and this voltage may be either a constant polarity voltage or an alternating polarity voltage.
This typical situation can be found, for instance, in the circuits disclosed in a copending application by T. E. Hendrickson, et al., entitled "Alternating Polarity Power Supply Control Apparatus" having serial No. 973,463 which employs, in the circuits disclosed therein, metal-oxide-silicon field-effect devices capable of controlling substantial amounts of power. Such devices are disclosed in a copending application by T. E. Hendrickson, et al., entitled "Semiconductor Apparatus" having Ser. No. 024,840. Both of these copending applications are assigned to the same assignee as the present application.
Further, the constant polarity, low voltage operated controlling circuit will often be in two parts with the first part remotely located from the electronic switching circuit, and with the second part immediately connected to and directing operation of the primary power transfer controlling element. In the example of the copending applications just referred to, this primary element would be the metal-oxide-silicon, field-effect device therein mentioned or described. Thus, relatively long electrical interconnection lines, or transmission lines, forming a transmission channel would be present for connecting the constant polarity, low voltage operated control system of the first part--usually a logic system--to the remotely located electronic switching control circuit of the second part which is immediately connected to the power circuit. Such long transmission lines provide the opportunity for substantial pickup of electrical noise which can disrupt proper control functions.
A possibility for reducing the chances that electrical noise pickup on the control lines will disrupt the control operation is based on providing dual electrical transmission lines for each control channel present. By having, as the remote electronic switching circuit, one which accepts commands through the channel as being valid only when command changes in signals occurring on one member of the pair of channel lines is accompanied by opposite changes in the signals present on the other member, noise pickup can be discriminated against relatively well. This is true because electrical noise pickup would result in common changes occurring in the signals on both lines; that is, the noise induced changes would be in the same direction.
Thus, to use this technique, an electronic switching circuit is required which would discriminate between (i) common changing signals occurring at a pair of signal inputs thereto, and (ii) opposite changing signals at these inputs. Such a capability would provide the basis for an initial circuit to generate control signals to command selected operations in the power circuit to which the discriminating circuit is to be connected. Furthermore, such an electronic switching system should provide electrical isolation between the power circuit and the initial circuit serving as the source of the two command input signals to prevent any interaction between this source and the power circuit.